Railroad hopper cars generally have one or more compartments for receiving dry granular, powdered or particulate materials. For some types of service the bottom of each compartment may be provided with an aeration assembly. The aeration assembly is similar in function to a discharge gate except it is not generally intended that the aeration assembly will be removed and replaced the way a discharge gate might be. The aeration assembly is constructed with angled or sloped bottom walls called slope sheets which join a discharge trough or conveyor. The trough has an unloading opening therein. The aeration assembly may also include end sheets and central divider sheets. Flanges around the perimeters of the aeration assembly components have holes for bolting the parts together. Similarly, the entire aeration assembly is bolted to a frame on the underside of the car body.
The slope sheets are tipped up from the horizontal to direct the lading to the unloading opening in the trough. Thus, the slope sheets function somewhat in the nature of a funnel. The funnel effect of the slope sheets is sufficient to make certain types of particulate lading flow freely out the opening by gravity. Other kinds of lading, generally finer powders, tend to bind up and clog the unloading opening. They will not flow by gravity and thus require some form of encouragement to depart from the car. This problem has been exacerbated by recent efforts to increase the payload of hopper cars by decreasing the angle of the slope sheets with respect to the horizontal. Decreasing the angle of the slope sheets increases the capacity of the cars but also decreases the tendency of the lading to flow out the unloading openings solely by force of gravity. Introduction of an aerating gas into the compartment to fluidize the lading in the vicinity of the unloading opening has been found necessary to induce flow in cases of fine powders or decreased slope sheet angle. The aerating gas is usually air but some materials may require use of an inert gas for safe handling. In the absence of an abnormal condition that causes caking in the area of the unloading opening, the aerated lading will flow out the opening like water.
Aerating the compartment requires some provision for introducing gas into the compartment in the area of the unloading opening and the slope sheets. Placing aerating gas supply pipes inside the compartments is not preferred primarily because the pipes themselves become an obstruction to the flow of lading. Further, interior supply pipes reduce the volume available for the lading and they are more difficult to service. Thus, the aerating gas supply pipes are located on the exterior of the compartment where they communicate with ports formed in the slope sheets.
The presence of the ports in the slope sheets leads to the need for a device in the nature of a check valve to cover the ports. The "check valve" has to prevent the lading from flowing through the ports and into the aerating gas supply pipes during loading, transportation and storage of the loaded car, while permitting aerating gas to flow through the supply pipes and into the lading during unloading. Rubber pads, sometimes known as flow cones, on the interior of the slope sheets and covering each port can be used for some types of service. These gas-impermeable pads have a diameter somewhat greater than the port and they are mounted so as to be biased into contact with the slope sheet where they normally cover and block the port. The pads will flex upwardly into the compartment when the pipes are pressurized to admit aerating gas into the compartment. While these rubber pads are effective for lading such as PTE, they are not suitable for food service such as flour, sugar and baking powder.
Food service requires an alternative arrangement for covering the ports. One approach that has been used for aerating is a permeable fabric pad. To increase dispersion of the gas throughout the slope sheet a metal separator plate has been placed between the fabric pad and the slope sheet. The separator is spaced from the slope sheet and has a plurality of apertures therein. The fabric pad sits on top of the separator. The aerating gas is introduced underneath the separator plate and flows up through the apertures and through the fabric pad into the lading. A later approach to slope sheet aeration dispensed with the separator plate and placed a fabric pad directly on the slope sheet. Fabric spacers sewn into the central portions of the pad lifted the pad off the slope sheet to increase gas dispersion.
One of the issues raised by the presence of the fabric pad is how to secure it in place. Since there were already mating flanges associated with the car body and aeration assembly, it made sense to extend the pad to this structure and clamp it between the mating flanges. However, it was immediately discovered that the pad not only transmits aerating gas normal to its surface, but parallel thereto as well. Gas leaked out the edges of the pad to the exterior of the compartment. This leakage could be so severe as to prevent proper pressurizing of the car for unloading. A messy, labor-intensive work-around was to apply a sealant to the edges of the pad after it was installed between the flanges. Another approach to solve the edge leakage problem was an extruded rubber gasket sewn to the perimeter of the pad. See U.S. Pat. No. 4,428,585. The corners of this gasket were mitered and glued. The gasket and pad were secured between the mounting flanges of the slope sheet and its supporting structures. Bolt holes extended through both the gasket and the underlying fabric. However, the pad still leaked aerating gas to atmosphere through the bolt holes, the sewn seams and the glued gasket joints. Washers surrounding the bolts provided only partial relief to the leakage problem.